The struck first at the post office on Church Street, Kimberworth, Rotherham, just after 5.30pm on December 29 and threatened staff with knives.
Post office workers fought off the men and they left empty-handed.
Less than an hour later, at 6.20pm, three men armed with knives entered the post office on Far Lane in East Dene, Rotherham.
They forced their way behind the counter, assaulted the shop keeper and made off with a large quantity of cash.Officers investigating the incidents believe the two robberies are linked and that the same men are responsible.
A CCTV image has been released of a man believed to hold vital information about the incidents.
Anyone with information should call South Yorkshire Police on 101 and quote incident number 792 of December 29 or Crimestoppers on 0800 555111.